1. Field of the Invention
The present invention relates generally to the field of electrophysiology. More particularly, this invention relates to methods and apparatus for mapping cardiac arrhythmias.
Symptoms of abnormal heart rhythm are generally referred to as cardiac arrhythmias, with an abnormally slow rhythm being classified as bradycardia and an abnormally rapid rhythm being referred to as tachycardia. The present invention is concerned with the treatment of tachycardias which are frequently caused by the presence of an "arrhythmogenic site" or "accessory atrioventricular pathway" close to the inner surface of one of the chambers of the heart. The heart includes a number of normal pathways which are responsible for the propagation of signals necessary for normal electrical mechanical function. The presence of arrhythmogenic sites or accessory pathways can bypass or short circuit the normal pathways, potentially resulting in very rapid heart contractions, referred to as tachycardias. Tachycardias may be defined as ventricular tachycardias (VT's) and supraventricular tachycardias (SVT's). VT's originate in the left or right ventricle and are typically caused by arrhythmogenic sites associated with a prior myocardial infarction. SVT's originate in the atria and are typically caused by an accessory pathway.
Treatment of both ventricular and supraventricular tachycardias may be accomplished by a variety of approaches, including drugs, surgery, implantable pacemakers/defibrillators, and catheter ablation. While drugs may be the treatment of choice for many patients, they only mask the symptoms and do not cure the underlying cause. Implantable devices only correct the arrhythmia after it occurs. Surgical and catheter-based treatments, in contrast, will actually cure the problem, usually by ablating the abnormal arrhythmogenic tissue or accessory pathway responsible for the tachycardia. The catheter-based treatments rely on the application of various destructive energy sources to the target tissue, including direct current electrical energy, radiofrequency electrical energy, laser energy, and the like.
Regardless of the particular catheter-based treatment selected, it will generally be necessary to initially map the interior surfaces of the heart to locate the arrhythmogenic site(s) and accessory pathway(s). Such mapping involves the measurement of electrical potentials at different locations within the heart to detect abnormalities using a procedure generally referred to as "programmed electrical stimulation." A catheter having a series of axially spaced-apart electrodes near its distal end is introduced to the interior of the heart, and pacing of the heart is induced using certain pairs of the electrodes, usually the distal pair. Progression of the induced signal within the heart is monitored using the remaining pairs of electrodes which are connected to conventional ECG monitoring equipment. In this way, the locations of the arrhythmogenic sites and accessory pathways can be generally determined. Usually, the locations will be more specifically determined prior to treatment using the treatment catheters themselves. Treatment catheters are usually steerable and have reference electrodes which can be more precisely positioned.
The reference electrode catheters used for ECG mapping have distal tips which are shaped to lie in preselected configurations within the heart chamber being mapped. It is thus necessary that the distal tip of the catheter be manipulable from the proximal end so that the distal tip can be properly oriented after it has been introduced to the chamber. In particular, the treating physician must be able to rotate the distal end of the catheter about its longitudinal axis by applying a rotational torque to the proximal end of the catheter. Thus, the reference electrode catheters must be torsionally stiff to transmit the encessary rotational force along their lengths. The ability to transmit torque along the catheter, however, must be achieved without significant loss of axial flexibility of the catheter, particularly at its distal end. It will be appreciated that the distal end of the catheter should remain soft and flexible in order to avoid injury to the heart. Previous catheter designs have attempted to meet these objectives but have not been completely satisfactory.
Thus, it would be desirable to provide improved reference electrode catheters having enhanced torsional stiffness without significant loss of axial flexibility, particularly at the distal tip region. Such catheters should have a relatively simple construction, with a reduced member of components and materials.
2. Description of the Background Art
A left ventricle mapping probe having a plurality of spaced-apart band electrodes is described in U.S. Pat. No. 4,777,955. A cardiac pacing catheter having a distal tip electrode and an electrically conductive torque member in a central lumen filled with solid polymer is described in U.S. Pat. No. 4,699,157. Intracardiac catheters for recording monophasic action potentials in a heart and including a distal tip electrode and one or more side electrodes are described in U.S. Pat. Nos. 4,979,510; 4,955,382; and 4,682,603.